As an emission type electronic displaying device, an electroluminescence device (ELD) is known. Elements constituting the ELD include an inorganic electroluminescent element and an organic electroluminescent element (hereinafter referred to also as an organic EL element).
Inorganic electroluminescent element has been used for a plane light source, however, a high voltage alternating current has been required to drive the element.
An organic EL element has a structure in which a light emitting layer containing a light emitting compound is arranged between a cathode and an anode, and an electron and a hole were injected into the light emitting layer and recombined to form an exciton. The element emits light, utilizing light (fluorescent light or phosphorescent light) generated by inactivation of the exciton, and the element can emit light by applying a relatively low voltage of several volts to several tens of volts. The element has a wide viewing angle and a high visuality since the element is of self light emission type. Further, the element is a thin, complete solid element, and therefore, the element is noted from the viewpoint of space saving and portability.
A practical organic EL element is required to emit light of high luminance with high efficiency at a lower power. For example, disclosed are an element exhibiting higher luminance of emitting light with longer life in which stilbene derivatives, distyrylarylene derivatives or tristyrylarylene derivatives doped with a slight amount of a fluorescent compound are employed (see Patent Document 1), an element which has an organic light emitting layer containing 8-hydroxyquinoline aluminum complex as a host compound doped with a slight amount of a fluorescent compound (see Patent Document 2), and an element which has an organic light emitting layer containing 8-hydroxyquinoline aluminum complex as a host compound doped with a quinacridone type dye (see Patent Document 3).
When light emitted through excited singlet state is used in the element disclosed in the above Patent documents, the upper limit of the external quantum efficiency (ηext) is considered to be at most 5%, because the generation probability of excited species capable of emitting light is 25%, since the generation ratio of singlet excited species to triplet excited species is 1:3, and further, external light emission efficiency is 20%.
Since an organic EL element, employing phosphorescence through the excited triplet, was reported by Princeton University (see Non-Patent Document 1), studies on materials emitting phosphorescence at room temperature have been actively carried out (see Non-Patent Document 2 and Patent Document 4).
As the upper limit of the internal quantum efficiency of the excited triplet is 100%, the light emission efficiency of the exited triplet is theoretically four times that of the excited singlet. Accordingly, light emission employing the excited triplet exhibits almost the same performance as a cold cathode tube, and can be applied to illumination.
For example, many kinds of heavy metal complexes such as iridium complexes have been synthesized and studied (see Non-Patent Document 3).
An example employing tris(2-phenylpyridine)iridium as a dopant has also been studied (see Non-Patent Document 2).
Other examples of a dopant which has been studied include, L2Ir (acac) such as (ppy)2Ir (acac) (see Non-Patent Document 4), tris(2-(p-tolyl)pyridine)iridium (Ir(ptpy)3), tris(benzo[h]quinoline)iridium (Ir(bzq)3) and Ir(bzq)2ClP (Bu)3 (see Non-Patent Document 5).
A hole transporting material is used as a host of a phosphorescent compound in order to increase emission efficiency (see Non-Patent Document 6).
Various kinds of electron transporting materials doped with a new iridium complex are used as a host of a phosphorescent compound (see Non-Patent Document 4). High emission efficiency has been obtained by incorporating a hole blocking layer (see Non-Patent Document 5).
Also disclosed is a thermally stable hole transporting material having an aromatic heterocyclic ring containing nitrogen as a partial structure and having a chemical structure extending in three or four directions from a nitrogen atom, or from an aryl portion as a center (see Patent Document 5). However, in Patent Document 5, an organic EL element has not been fully disclosed.
Further, an aromatic heterocyclic compound containing nitrogen, which is a light emitting compound exhibiting high luminance, has been disclosed (see Patent Document 6), however, an organic EL element exhibiting phosphorescent emission has not been fully disclosed in Patent Document 6.
At present, an organic electroluminescent element emitting phosphorescence with further higher emission efficiency and longer life has been studied.
An external quantum efficiency of around 20%, which is a theoretical threshold, has been attained in green light emission in a lower current region (a lower luminance region), however, the theoretical threshold has not been attained in a higher current region (a higher luminance region). Further, a sufficient emission efficiency has not been attained in other color emission, where further improvement is required. With respect to the practical use in the near future, required is an organic EL element which efficiently emits light with high luminance at a lower power. Specifically, an organic EL element which emits a blue phosphorescence with high efficiency is required.
Accordingly, an object of the present invention is to provide an organic electroluminescent element and a display device exhibiting high luminance and, furthermore, to provide an organic electroluminescent element and a display device exhibiting long life.
(Patent Document 1)                Japanese Pat. No. 3093796        
(Patent Document 2)                Japanese Patent Publication Open to Public Inspection (hereafter referred to as JP-A) No. 63-264692        
(Patent Document 3)                JP-A No. 3-255190        
(Patent Document 4)                U.S. Pat. No. 6,097,147        
(Patent Document 5)                Examined Japanese Patent Publication No, 7-110940        
(Patent Document 6)                JP-A No. 2001-160488        
(Non-Patent Document 1)                M. A. Baldo et al., nature, 395, 151-154 (1998)        
(Non-Patent Document 2)                M. A. Baldo et al., nature, 403(17), 750-753 (2000)        
(Non-Patent Document 3)                S. Lamansky et al., J. Am. Chem. Soc., 123, 4304        
(Non-Patent Document 4)                M. E. Tompson et al., The 10th International Workshop on Inorganic and Organic Electroluminescence (EL '00, Hamamatsu)        
(Non-Patent Document 5)                Moon-Jae Youn.0g, Tetsuo Tsutsui et al., The 10th International Workshop on Inorganic and Organic Electroluminescence (EL '00, Hamamatsu)        
(Non-Patent Document 6)                Ikai et al., The 10th International Workshop on Inorganic and Organic Electroluminescence (EL '00, Hamamatsu)        